Stimulation of the T cell antigen receptor initiates a complex cascade of intracellular events to elicit T cell function. Phosphorylation of tyrosine residues within a conserved ITAM motif found in all immunoreceptors and present on the TCR-associated CD3 polypeptides is the initiating event. Although it is clear that three major signaling pathways (Ras, PI3K, and PLCg) emanate from ITAM phosphorylation, the details of how these pathways are initiated and maintained is not understood. This is particularly true of the PLCg pathway; most of the work regarding its activation and regulation has been done in non-lymphoid systems. PLCg hydrolyzes investigator-4,5P2 to generate IP3 and DAG. Both these products have demonstrated mediator function in lymphoid cells; IP3 increases cytosolic Ca2+ levels and DAG activates certain protein kinase C isoforms. Genetic studies have established that PLCg is phosphorylated by members of the Syk/ZAP-70 and the Tec-kinase families, and both phosphorylation inputs seem to be essential for PLCg activation. There are two isoforms of PLCg (1 & 2); T cells predominantly express PLCg1. PLCg1 contains a pleckstrin homology domain and a split catalytic domain, separated by a N- and C-terminal SH2 domains and a SH3 domain. In other systems, PLCg1 is tyrosine phosphorylated within a about50 residue region between the C-terminal SH2 and the SH3 at tyrosines 771 and 783, as well as a C-terminal tyrosine residue 1254; phosphorylation of the latter site appears to be irrelevant for enzyme regulation. However, experiments in fibroblasts stimulated with PDGF indicate that phosphorylation at residue 783 is necessary for increased enzymatic activity, while phosphorylation at Y771 results in inhibition of activity.